A step leading to muscle contraction is the intracellular release of Ca2 ion. My objectives are (1) to compare some physical and chemical events that indicate Ca2 ion release in contracting muscle, (2) to describe the effects of agents that alter Ca2 ion release during muscle contraction, and (3) to determine the mechanism of Ca2 ion release and the role of calcium in influencing mechanical properties of amphibian skeletal muscle. Physical and chemical features indicative of Ca2 ion release will be correlated with changes in membrane potential, charge movements, and tension development of isolated intact cells. The specific physical features are striation spacing and myofibrillar orientation recorded by visible light microscopy and cine-photography. The chemical feature is the relative change in intracellular (Ca2 ion) recorded as light emission from cells microinjected with the Ca2 ion-sensitive photoprotein aequorin. The presence or absence of correlations among these variables will be used to evaluate the actions of agents that may change intracellular Ca2 ion release, and to evaluate models for stimulus-response coupling; e.g., is Ca2 ion release and tension development after brief depolarization unequivocally proportional to the "mechanically effective period", that is, proportional to some relationship that includes both the amplitude and duration of change in membrane potential? Does the amplitude and rate of decay of an aequorin signal faithfully indicate the changes after brief depolarization? Is Ca2 ion release and contracture after long depolarization proportional to a different relationship where one must surmise that changes in things such as cyclic nucleotide concentration or Ca2 ion-sensitive phosphorylation reactions control mechanical properties? Is spontaneous relaxation during maintained depolarization due to exhaustion of the store of releasable Ca2 ion, or does Ca2 ion release inactivate with time? Are slow non-ionic charge movements unequivocally associated with Ca2 ion channel gating? Are modifications in aequorin signals best described by Winegrad's model in which there is a stimulus-dependent depletion of Ca2 ion in release sites and progressive displacement of Ca to uptake sites in the SR, or by a model in which Ca2 ion-permeable pores inactivate in agreement with Armstrong's blocking particle hypothesis?